Normal human peripheral blood lymphocytes (PBL) are induced by immobilized IgG-antigen complexes or aggregate to lyse various erythrocyte target cells. This reaction is modulated by suppressor cells also present in normal human PBL populations. The suppressor cells are T cells. The suppressive effect of T cells is alleviated by incubating T cells with monocytes for 8-18 hours. The effector cell has been characterized as a complement receptor positive lymphocyte (CRL plus). Preliminary evidence indicates that the CRL plus cell secretes a soluble factor(s) which is absorbed by nucleated cells and by erythrocytes. This factor(s) lysis erythrocytes and inhibits tumor cell DNA biosynthesis as measured by 3H-TdR incorporation. We propose to further characterize this cytolytic mechanism.